POS0238 IMPLICATION OF THE CSF1-CSF1R PATHWAY IN THE CROSSTALK BETWEEN SYNOVIAL FIBROBLASTS AND MACROPHAGES IN PIGMENTED VILLONODULAR SYNOVITIS
Authors: R. Dallatorre, J. De Lima, F. Blanchard, B. Le Goff
Journal: Annals of the Rheumatic Diseases (2023)

<h3>Background</h3> Pigmented villonodular synovitis (PVNS) is a rare joint disease with an estimated incidence of 14 per million per year, arising from synovia that results in a secondary inflammatory joint response, pain and destruction. PVNS is associated with abnormal fibroblastic cells, often involving a specific chromosomal translocation, t(1;2) (CSF-1;COL6A3), resulting in the overexpression of colony-stimulating factor 1 (CSF1 or M-CSF), recruitment of CSF1 receptor (CSF1R) macrophages and fusion in giant cells. Despite therapeutic option such as surgery, the risk of recurrence of the synovitis remains high (50%). This has led to the development of systemic therapies targeting the CSF1 pathway (kinase inhibitors such as Imatinib, monoclonal antibodies) that have shown their efficacy is about 30% of patients but with significant and sometimes serious adverse effects. Therefore, there is currently a clear medical need to improve the management of patients with PVNS. <h3>Objectives</h3> To better decipher the mechanisms leading to the development of PVNS and study the crosstalk between synovial fibroblasts and macrophages in this disease. <h3>Methods</h3> Synovial tissue from newly diagnosed PVNS (diffuse or localized forms) patients was used for this analysis. Histology H&amp;E staining and immunohistochemistry was performed. Single cell analysis was performed on 5 patients. We implemented a primary culture of synovial fibroblasts from PVNS biopsies stimulated with pro-inflammatory cytokines. We studied the effect of culture supernatant on blood derived monocytes/macrophages. <h3>Results</h3> Immunohistochemistry revealed the presence of synovial PDPN+ fibroblasts, CD31+ endothelial cells, Ki67+ proliferating cells, MERTK+ and CCR2+ infiltrating the tissues. We performed single-cell RNA sequencing (n=5 patients). Combined with public scRNAseq datasets, we observed that PVNS (n=8) lining PDPN+PRG4+ fibroblasts are the main sources and express higher levels of CSF1 than in osteoarthritis (OA, n=3) or rheumatoid arthritis (RA, n=4). All macrophage subsets (tissue resident TIMD4+ or LYVE1+ or infiltrating inflammatory macrophages) highly express CSF1R that could explain their enrichment (2 folds, p&lt;0.05), especially in the localized forms of PVNS. Ratio between fibroblasts and macrophages was increase in the diffuse forms of the disease. We next study the in vitro crosstalk between fibroblasts and macrophages. For this purpose, we analysed conditioned media (CM) from PVNS synovial fibroblasts (n=3). After treatment with TNFa or IL1b, enhanced secretion of CSF1 was observed and the CM had an enhanced capacity to sustain viability of blood derived monocytes/macrophages (2-3 folds, p&lt;0.05). In this setting, the potent CSF1R inhibitor Imatinib totally reversed macrophage viability to baseline (p&lt;0.05). <h3>Conclusion</h3> This study highlights the key role played by synovial fibroblasts and macrophages in PVNS pathogenesis. An inflammatory vicious cycle takes place between these two cell types, implicating CSF1-CSF1R. Efficient targeting of this pathway is the subject of current research and we already validated a PVNS mouse model using xenografted PVNS human biopsies that will be useful to study for example the local delivery of CSF1R kinase inhibitors or neutralizing antibodies. <h3>REFERENCES:</h3> NIL. <h3>Acknowledgements:</h3> NIL. <h3>Disclosure of Interests</h3> None Declared.